Last Saturday, several thousand scientists and their supporters massed in front of the Treasury building in Westminster to speak out against proposed funding cuts for scientific research. Standing on the stage for my opening speech, I surveyed the sea of protestors in a state of awe.

It was past the starting time of 2pm, but people were still streaming into King Charles Street from both ends of the road. I could see people of all descriptions: famous scientists, young students, families with small children. Many people sported white coats and held up placards or colourful accessories: a foam model of Jupiter; a buckyball on a stick; the international symbol for toxic irritants with a photo of Vince Cable superimposed within the yellow triangle. The mood was well-behaved and upbeat, but the opening cheer echoed with a mighty roar, driving home just how formidable people can be when many act as one.

It had been only a month since I wrote a blog post proposing that scientists take to the streets – four short weeks from a crazy idea to its culmination. Along the way I received a whirlwind education in politics and grassroots organisation. My colleagues and I might be good at splicing genes or peering into the depths of the universe, but how many scientists does it take to assemble 300 placards in four hours while being faintly high on spray glue? (Answer: about a dozen.) These lessons and others occurred in a haze of distracted days and late nights, and go some way toward explaining the complaint that more scientists don't engage in policy activism: if they did, at least on this scale, research would grind to a halt.

The big day finally arrived. Over the next two hours, speakers entertained the crowd in a line-up aimed at stimulating both hearts and minds. Colin Blakemore, Oxford neurobiologist and former head of the Medical Research Council, and Imran Khan, director of the Campaign for Science and Engineering, made the detailed case for the importance of research investment to maintain economic growth. Bad Science columnist and medic Ben Goldacre spoke about the perils of a cuts-fuelled brain drain, and Simon Denegri, chief exec of the Association of Medical Research Charities, explained how public funding is crucial for supporting its mission. Sex educator Petra Boynton described why cutting basic research would imperil collaborations with social scientists working in the developing world. Paul Noon, general secretary of Prospect, spoke on behalf research trade unions, while Michael Brooks explained why he set up a political party devoted to science.

On a more personal note, cancer survivor Claire Daniels and former Alzheimer carer Vivienne Hill gave moving personal takes on the importance of research.

But it wasn't all sober and serious. Materials scientist and broadcaster Mark Miodownik of King's College London spoke about the more inspirational aspects of science as a cultural endeavour, and stand-up science comics Timandra Harkness and Dean Burnett easily scored laughs. But Evan Harris, former MP and tireless champion for evidence-based policy, arguably stole the show with his singing and chanting stints – which are rapidly becoming YouTube classics. Crowd-sourced videos and images of all the speakers and antics have been flooding in.

20 October will be the moment of truth, however. When the comprehensive spending review is announced, we will finally find out whether our efforts made a difference. But there is one thing, at least, about which we can already be certain: scientists in the UK will no longer take things lying down. The slumbering beast truly has awakened.

This article was amended on Tuesday 12 October 2010. The original stated that the Science is Vital petition "closes at 13:30 BST today". This has been corrected.